1. Field of the Invention
The invention is in the field of electronics and more specifically in the field of signal-processing systems.
2. Related Art
A transconductor is a circuit configured to generate an output current proportional to an input voltage. Transconductors are commonly used for signal detection in applications such as communications, sensors, signal processing, et cetera. High linearity between the input voltage and output current is important to many applications. Transconductors are characterized by the frequency range and voltage range over which a linear output is produced.
FIG. 1 is a conceptual illustration of a Transconductor 100 of the prior art. Transconductor 100 includes a differential voltage input 110 at which voltages Vinp (input p) and Vinn (input n) are applied. These voltages are received by a Buffer 120. Buffer 120 is an active circuit used to reproduce the difference between Vinn and Vinp across a Resistor 130 of resistance R. This results in a current iR through Resistor 130. The current iR is replicated by a Current Source 140 that provides a current proportional to the difference between Vinn and Vinp at an Output 150.
Transconductor 100 may be limited in the range of input voltages that can converted to a linearly proportional output current. For example, if the voltage difference |Vinn−Vinp| is close to or greater than a supply voltage (Vcc) of Buffer 120, then Transconductor 100 may not be able to reliably detect these voltages. One solution to this problem is to convert the input voltage signal into a current signal relative to a “virtual ground” before it enters the active section of the transconductor circuit. The differences between each of Vinn and Vinp and the virtual ground determine the values of the current signals. In this approach, the difference between each of Vinn and Vinp and the virtual ground is not limited to being less than Vcc. The virtual ground is preferably set close to the input “common-mode voltage.” The common-mode voltage is the voltage that Vinn and Vinp have in common and can be defined as Vincm=(Vinp+Vinn)/2.
FIG. 2 is a conceptual illustration of a Transconductor 200 of the prior art including a virtual ground voltage (Vvg). The virtual ground is established by a Voltage Source 210 configured to provide a potential Vvg relative to a Common Ground 220. Vinp and Vinn are applied to a Resistor 230 and a Resistor 240, respectively, which are characterized by resistances Rp and Rn. A current iRp passes through Resistor 230, where iRp=(Vinp−Vvg)/Rp. Likewise, a current iRn passes through the Resistor 240, where iRn=(Vinn−Vvg)/Rn. A Current Source 250 is used to replicate iRp at an Output 270 and a Current Source 260 is used to replicate iRn at an Output 280.
The voltage at the virtual ground Vvg is kept constant over a range of possible values of current inputs (iRp and iRn). The combined currents through the input resistors Rp and Rn can be expressed in terms of a common-mode voltage (Vincm) and a differential-mode voltage (Vindiff) as follows:
                              i          rp                =                                            V              indiff                                      2              ⁢                              R                p                                              +                                                    V                                  in                  ⁢                                                                          ⁢                  cm                                            -                              V                vg                                                    R              p                                                          Equ        .                                  ⁢        1                                          i          rn                =                                            V              indiff                                      2              ⁢                              R                n                                              +                                                    V                                  in                  ⁢                                                                          ⁢                  cm                                            -                              V                vg                                                    R              n                                                          Equ        .                                  ⁢        2            where Vincm=(Vinp+Vinn)/2 and Vindiff=Vinp−Vinn.The output currents ioutp and ioutn are, therefore, a sum of a component proportional to the signal of interest (the differential voltage Vindiff) and an undesired common-mode component (Vincm−Vvg).
In addition to the presence of the undesired common-mode component, a problem with the approach illustrated in FIG. 2 is that Vvg should be as close to the input common-mode voltage as possible. This can require a costly off-chip voltage source. Further. if the input voltages include high frequency components, Vvg must track these components . This is difficult to do using an external voltage source or when there are other sources of high frequency noise, and places further costs and restrictions on the system.
In some cases AC coupling is used to eliminate a DC common-mode voltage. However, this approach may require costly external components or limit the frequency bandwidth of the system.
Transconductors are sometimes used in systems in which the strength of incoming signals may vary considerably. For example, in computing networks, electrical or optical signals may be attenuated by different amounts depending on factors such as the distance traveled from a source. As such, the peak voltages at a receiver may vary by several decades, e.g. 70 dB or more. In applications requiring a high data throughput it may also be desirable to use fast low-voltage circuitry, such as CMOS circuits having a maximum supply voltage of 3.3 Volts or lower. This places further limitations on the transconductor.